With the widespread use of smart phones and wearable devices, there has been a demand for detecting character strings from images existing in real space such as character strings on signboards, signs, and menus in restaurants photographed by a camera. The character strings in images photographed by a camera vary in the way they look depending on the lighting conditions of the character strings and the effects of shadows. As a method of detecting character strings from such images, there are, for example, a technique using a connected component which is obtained by connecting pixels in an image, or a technique using a detector based on mechanical learning.